Pass element is IRFBC30 and error amp. is BUH51. I chose them because the Vout is high, 405V. Vin is 450V.

Operation current is about 40 mA.

It is a simple two transistor voltage regulator. Upper resistor is 81K and lower resistor is 120K for the voltage divider in the output. Voltage reference is 235V, using 3 zenor diodes in series connected to emittor of BUH51.

hfe of BUH51 is about 27, based on measurement.

A 100 ohm resistor is connected between G of IRFBC30 and collector of BUH51.

A 10K3 resistor is connected between Vin (that is D of IRFBC30) and collector of BUH51.

This regulator is for a tube power amp.

When I powered it up without preamp, it is stable, Vout is about 407V.